1. Field of the Invention
This invention relates to a device for electrical stimulation and signal derivation from neuronic tissue. More accurately stated, this invention relates to a multi-channel neuro-electrode for stimulating neurons in interior neuronic layers, or interior fibers of a nerve, which is suitable for extended time applications because of a particular stability.
2. Discussion of Related Art
During development of neuro-implants for correcting handicaps such as loss of hearing or blindness, a practical possibility for correcting further neurological handicaps, such as paraplegic handicaps, was recorded. While the first cochlear and retinal implants use sensory neurons in the sensory organs, such as the cochlea or retina, for transmitting information, in other applications the requirement exists for making a direct connection with neurons of different cortical regions. In most cases, these are located relatively deeply underneath the cortical surface and are difficult to access.
Two types of electrodes are known for making a connection with cortical neurons located a few millimeters beneath the surface: surface electrodes and penetrating electrodes. At present, cortical implants on the market have employed surface electrodes, which are placed over a target area and fastened. The main disadvantage of this solution is that very high currents are required for the stimulation of neurons in order to address the deeply located neurons.
A number of penetrating electrodes having several needle-like electrodes was developed as an alternative. An electrode array is described in U.S. Pat. No. 5,215,088, by which electrical pulses can be conducted out of interior cortical layers. The employment of these arrays as neuro-stimulators discloses some disadvantages of this solution: electrical losses in each electrode increase because of the use of doped silicon as the electrode material, because impedance can lie within the range of 300 to 800 KOhm. Therefore, energy consumption of the neuro-implant increases on the one hand and, on the other hand, the thermal stress on the nervous tissue in the vicinity of the electrodes increases. A further problem is posed by the manufacture of electrode profiles, such as shown in FIG. 1. Such a profile is required for the stimulation of different cortical layers which, for example, are responsible for various sensory representations of the image in the visual cortex.
A further embodiment of penetrating electrodes is described in US Patent Application 2003/0176905. Although this embodiment has excellent electrical properties, it is only suitable for acute employment, because it is not possible to construct the electrodes in a fully implementable manner because of the enormous size of the connecting elements. At the same time, the stiffness of the described electrodes is not sufficient for penetrating the nerves.
The stability over extended time causes another common problem of neuro-electrodes. Because of an interaction with biological tissue, a sort of a protective layer of neuroglial cells grows on the electrodes, which leads to a clear increase in stimulation thresholds and to the worsening of the signal quality. Investigations by Branner et al (2004) have shown that approximately 20% of the electrodes can no longer be used after 6 months in the body. Because the desired service life of neuro-implants is more than 10 years, this can result in the complete failure of the neuro-stimulator.